A persistent problem with goggles and with spectacles intended to provide eye protection is their tendency to mist up even under conditions of use that are not particularly arduous. Naturally, the tendency to mist is higher when the wearer is hot and the environment is either cold or damp.
Eye protectors which are substantially enclosed, are frequently provided with ventilating apertures at various positions around their frame periphery but generally these are too small to provide adequate ventilation. For instance, British Patent Specification No. 603931 discloses goggles of the kind having separate eye cups with each eye cup having several small rearward facing cowled openings. The total ventilator area is very small.
In many circumstances, the size of the ventilators is limited by the protective function the eye protectors are required to perform. In an attempt to provide a sufficient level of ventilation without providing too direct a path for the ingress of dangerous materials, turret ventilators, labyrinth ventilators or mesh ventilators are employed. However, we have found that these do little to resist misting, probably because the flow path for air entering and leaving the eye protectors is either too convoluted or too small, giving rise to excessive resistance to flow, or produces an air flow pattern in which air travels around the turret or labyrinth and leaves again without effectively sweeping through the eye protector.
Our research indicates that in order to prevent misting an airflow through an eye protector such as a pair of goggles needs to be at the level of 1 liter to 8 liter per minute, the actual flow rate depending upon the operational circumstances prevailing. In normal indoor working environments ambient wind speeds are not greater than 0.1 to 0.2 m per second since higher wind speeds are likely to cause irritation. In order to obtain an adequate flow rate of air, say 3 liters per minute, through goggles at a linear wind speed of say 0.1 m per second the theoretical unrestricted area of access into and subsequently out of goggles or spectacles must be about 500 mm.sup.2 per side. A level of over 1000 mm.sup.2 per side of the goggles or spectacles would be desirable.
We have found that existing goggles that provide adequate protection do not provide this level of ventilation. Existing goggles often provide areas of ventilation covered with wire mesh or plastics grill which is inadequate often because of the small size of the individual holes, even if their aggregate area is fairly high. The air flow through a mass of small apertures suffers more resitance than through a larger aperture of the same total area. Except in turbulent conditions, the further the air is from the edge of the aperture through which it flows, the faster it flows. In eye protectors with a large number of small perforations, much of the air flow is slowed by being close to the walls of the perforations.
Moreover, it is often the case that some of the air holes provided are cone shaped with only small exits at the end, some of which are completely sealed over because of poor moulding techniques.
Naturally, it is not possible simply to increase the size of the ventilating holes without rendering eye protectors such as goggles liable to admit foreign material in an undesirable manner.
Another method of avoiding misting which has been proposed in the past is double glazing the lens or lenses of goggles and spectacles. This may help by raising the temperature of the inner lens but because the double glazed lens unit is thicker overall, it may be that the inner lens is brought closer to the face of the wearer thus decreasing the space within the eye protector through which air may flow. This may even make misting worse rather than better.
Anti-mist treatments for lenses or anti-mist compounds which may be applied to lenses do not prevent condensation. They either convert the condensed moisture into a wet rather than a misted layer or absorb the moisture for a short time. This is only satisfactory where there is an intermittent air flow which at its higher level is sufficient to remove the moisture from the lens.
Accordingly, there is a continuing need for a system of ventilation providing large enough, unobstructed apertures to provide adequate ventilation whilst at the same time providing sufficient protection against penetration of dangerous materials into the goggles in common work situations.